tests/bios-tables-test: check the value returned by fopen()

The function fopen() may fail, so check its return value.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Li Liu <john.liuli@huawei.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

tests/bios-tables-test.c

@@ -790,6 +790,11 @@ int main(int argc, char *argv[])
     const char *arch = qtest_get_arch();
     FILE *f = fopen(disk, "w");
     int ret;
+
+    if (!f) {
+        fprintf(stderr, "Couldn't open \"%s\": %s", disk, strerror(errno));
+        return 1;
+    }
     fwrite(boot_sector, 1, sizeof boot_sector, f);
     fclose(f);